Monocrystalline semiconductor wafers are usually cut from single crystals obtained by semiconductor material melted inductively or in a crucible gradually being crystallized on a rotating seed crystal. A virtually round ingot having a central longitudinal axis as a geometrical axis arises in the process. Afterward, the ingot is reworked to form one or more cylindrical blocks and semiconductor wafers having a specific crystal orientation are sliced from these blocks. The reworking generally comprises slicing the block from the single crystal along cutting planes perpendicular to the central longitudinal axis of the single crystal and cylindrically grinding the block around the central longitudinal axis to form a block having the form of a cylinder.
Wire saws are usually used for slicing the semiconductor wafers. Inner-diameter saws are also suitable, in principle, although their throughput is lower. Wire saws have a wire web formed by wires lying parallel. In the course of the sawing operation, the wires penetrate through the block, as a result of which a number of semiconductor wafers corresponding to the number of gaps between the wires penetrating through the block arise simultaneously. In order to optimize the throughput, the wire web available should be utilized as completely as possible. Therefore, it is often the case that two or more blocks are arranged one behind another and sawn simultaneously.
The customer for the semiconductor wafers demands a specific orientation of the crystal lattice. The normal to the surface of the front side of the semiconductor wafer is intended to lie parallel to a vector representing the specific orientation of the crystal lattice, this being referred to hereinafter as the sought orientation of the crystal lattice.
If the central longitudinal axis of the block represents the sought orientation of the crystal lattice, semiconductor wafers having the sought orientation of the crystal lattice arise if, in the process of slicing the semiconductor wafers, the cutting planes through the block are placed perpendicularly to the central longitudinal axis of the block.
There are reasons, however, on account of which single crystals are produced with a misorientation. A misorientation of the single crystal is present if the central longitudinal axis of the single crystal does not represent the sought orientation of the crystal lattice of the semiconductor wafers, but rather forms an angle θ with a crystallographic axis representing this orientation. That can happen unintentionally, for example if the seed crystal is already misoriented or the misorientation arises during the process of pulling the single crystal, or intentionally, for example if the misorientation is brought about in order that dislocations can be better eliminated, which is often performed in practice in order to produce (110)-oriented semiconductor wafers composed of silicon.
In accordance with the method described in JP2000323443 A2, firstly a block is sliced from the single crystal, the cutting planes being placed perpendicularly to the central longitudinal axis of the single crystal. This is followed by grinding the circumferential surface of the block around the crystallographic axis representing the sought orientation of the crystal lattice of the semiconductor wafers. The ground block has the form of an oblique cylinder, the end surfaces of which do not lie perpendicular to said crystallographic axis. The advantage of this method is that correctly oriented round semiconductor wafers arise if the cutting planes during the process of slicing the semiconductor wafers from the block are oriented perpendicularly to the axis representing the sought orientation of the crystal lattice of the semiconductor wafers. One disadvantage of this procedure is that products having a wedge-shaped cross section arise at the end sides of the block, which products, as waste, lower the yield of the method.
In accordance with the method described in EP 1 498 516 A1, firstly a block is sliced from the single crystal, the cutting planes being placed perpendicular to the central longitudinal axis of the single crystal. This is followed by grinding the block around the central longitudinal axis, said block acquiring the form of a cylinder. The end surfaces of the ground block lie perpendicular to the central longitudinal axis thereof. During the process of slicing the semiconductor wafers, the ground block is oriented such that the cutting planes lie perpendicular to the crystallographic axis representing the sought orientation of the crystal lattice of the semiconductor wafers. One disadvantage of this method is that products having a wedge-shaped cross section arise at the end sides of the block, which products, as waste, lower the yield of the method. A further disadvantage is that the orientation of the block has to be effected in the wire saw. The cutting of a plurality of short blocks in one cut is not possible, and the orientation of the block in the wire saw is complicated and susceptible to faults. What is also disadvantageous is that the sliced semiconductor wafers are not round, but rather have an oval form, owing to the position of the cutting planes.